1. Technical Field
The present disclosure relates to a rotary machine provided with an atmosphere relief mechanism for preventing abnormal pressure rise in an interior space of a casing and also relates to a mounting method of the atmosphere relief mechanism.
2. Description of the Related Art
A rotary machine such as a steam turbine and a gas turbine includes a casing for housing a rotor so that working fluid (inner fluid) is hermetically sealed in an interior space of the casing. During normal operation of the rotary machine, normally there is a pressure difference between the interior space of the casing and an atmospheric side. For instance, some low-pressure casings of the steam turbine are configured so that a rotor being rotated upon receiving steam is covered by an inner casing and the rotor and the inner casing are covered by an outer casing. In this type of configuration, the casing forms an outer shell of the outer casing and the interior space of the casing functions as an exhaust chamber. The exhaust chamber is provided to introduce the steam (exhaust gas) having driven the rotor to a condenser and is maintained at negative pressure during normal operation of the steam turbine.
However, if the steam leaks from a piping or there is pump failure in a negative-pressure system arranged from the outer casing to the condenser, etc., the pressure in the interior space of the outer casing can increase abnormally. If this pressure exceeds atmospheric pressure, this causes abnormality in operation of the steam turbine and the condenser due to temperature rise of the steam and change in the pressure direction from external pressure to internal pressure, thereby making it inevitable to stop the operation. To avoid this, an atmosphere relief mechanism is provided to discharge the steam in the outer casing to the atmosphere when the exhaust pressure exceeds the atmospheric pressure.
Generally, the atmosphere relief mechanism used in the rotary machine is provided with a rupture disc configured to rupture when the pressure in the interior space of the casing exceeds a predetermined pressure. The rupture disc is configured to rupture when the pressure rises abnormally. Once the rupture disc ruptures, the interior space of the casing communicates with the atmosphere side to relieve the pressure of the interior space. Normally, the rupture disc is fixed by a holder to an opening for relieving to the atmosphere, which is formed in the casing. During a normal operation of the rotary machine, the opening is closed by the rupture disc to isolate the interior space from the atmosphere side.
As a related technique, disclosed in Japanese Unexamined Utility Model Application Publication No. 62-81771 is a mounting device for a floating disc for atmosphere relief provided in the steam turbine. This mounting device is configured to place the atmosphere relief disc made of a lead sheet at the opening of the casing and then fasten outer edge of the atmosphere relief disc to the casing by a holding disc and a bolt. Inside the atmosphere relief disc, the floating disc is provided so that, when the pressure inside the casing increases, the floating disc moves toward the atmosphere side and then causes shear failure of the lead sheet at the outer edge of the floating disc.
Further, in Japanese Unexamined Utility Model Application Publication No. 2-126001, as a rupture disc used in an exhaust casing of the steam turbine, metal plates made of stainless steel, lead, nickel aluminum, etc. are described.
In the case of using the lead plate as the atmosphere relief disc as described in JP 62-81771 and JP 2-126001, lead has a low elastic coefficient and thus the lead plate is deformed even by a pressure change that does not reach the pressure at which the lead plate ruptures, and repeating of this deformation can cause a bolt hole provided in the lead plate for fastening to stretch. Thus, sealability of the atmosphere relief mechanism declines and the air may enter the interior space of the casing which is maintained at negative pressure during the normal operation. In order to prevent this, the lead plate must be replaced frequently. Moreover, the material of the atmosphere relief plate, lead, is known to be harmful to humans and thus there is a trend to restrict use of this material.
In view of this, an atmosphere relief disc having stacked layers was proposed. The atmosphere relief disc is formed by stacking thin plates made of material such as SUS and fluorine resin.
For instance, described in Japanese Unexamined Patent Application Publication No. 8-226308 is to arrange a rupture disc in an exhaust gas duct for leading exhaust gas from a gas turbine to a boiler furnace. This rupture disc is manufactured by stacking thin plates made of material such as graphite, SUS 316 and Teflon® and is supported between a pair of flange portions formed in the duct.